As...The default setting of Windows causes a slight delay when opening the sub categories of the start menu.....That is why we need to hover the mouse on it for about half a second.....You can change this default setting by following these steps.....

1. Go to start==> run==> Type regedit. Press enter.....

2. Navigate to "HKEY_CURRENT_USER----> Control Panel."

3. Click on Desktop.

4. On the right pane look out for 'MenuShowDelay'....(If you can't find it right-click and create a new 'Multi-String Value' and rename it as 'MenuShowDelay')

5. The default setting is 400....

6. Double click it to modify it....

7. Set it to 0 and there will be no delay...

8. Exit registry and reboot.

However making it zero makes it difficult to move the mouse without accidentally opening other folders. So try giving some lower values and choose the one which best suits you. ..